Dispersion compensating waveguide for optical transmission systems

ABSTRACT

In order to use one dispersion compensating fiber element for selecting a given value of dispersion, one or more frequency or wavelength dependent optical reflection gratings (G 1 , G 2 , G 3 ) is located at such a position along the unit ( 8 ) that the double traversal of a section results in a desired value of dispersion at a frequency. A directional coupler ( 9 ) diverts the reflected wave to utilization means ( 10 ) for its recovery. If a different value of dispersion at the same wavelength, or if some value of dispersion at a different frequency, is required, a reflection grating effective to reflect at the appropriate frequency and at the appropriate position, gives the required values.

BACKGROUND OF THE INVENTION

The present invention relates to a dispersion compensating waveguide(DCW), which has the property of introducing frequency dispersion intransmitted optical waves and is generally used for compensatingunwanted dispersion in a transmission path usually comprising fibre. Theinvention further relates to an optical transmission systemincorporating a dispersion compensated waveguide.

Typically the waveguide will comprise dispersion compensating fibre((DCF).

In order that a required dispersion be introduced, without a requirementfor unduly long lengths of dispersion compensating fibre, the fibre maybe highly doped, e.g. germania doped silica. Whether or not it is highlydoped, it is usual to cut or otherwise select an appropriate length toselect a dispersion value, which is somewhat inflexible. If more thanone wavelength is being used in the transmission of information, therewill be an inevitable trade-off in throughput resulting from selectingan optimum length to suit all the wavelengths used.

It would be also desirable if losses could be compensated independentlyof the compensation of dispersion, or so as to enable use of dispersioncompensating fibre to be more flexible.

SUMMARY OF THE INVENTION

A particular arrangement to be described below as being helpful inunderstanding the invention proposes the provision of Bragg gratings inthe dispersion compensating fibre, and makes use of the reflectedsignals from the gratings. The incident signal traverses selectivelydifferent lengths of the dispersion compensating fibre according to thedispersion required by the appropriate positioning of the grating alongthe dispersion compensating fibre.

If various selected dispersions are required, a prior method is to usedispersion compensating fibres of different lengths, or to cut down froma starting length. This embodiment uses reflection gratings at intervalssuch that different (double—) lengths of the dispersion compensatingfibre can be selected for traversal by the signal, and hence differentdispersions can be selected. The selection would be achieved in practiceby splicing or writing a grating into the fibre at the required lengthdown the fibre.

In the specification of U.S. Pat. No. 5,404,413 there was proposed anoptical circulator with three ports. The first and third ports wereconnected to optical fibre systems. The second port was connected to adispersion compensating fibre and return means. A signal passed throughthe dispersion compensating fibre twice, thus permitting the use ofshorter compensating fibres than previously.

The following description and drawings disclose a previously proposedarrangement and, by means of examples, the invention which ischaracterised in the appended claims, whose terms determine the extentof the protection conferred hereby.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a schematic diagram illustrating pulse transmission through apreviously proposed optical waveguide in series with a dispersioncompensating fibre element for use in explaining the invention;

FIG. 2 shows pumping of a dispersion compensating fibre element toenable Raman amplification which may optionally be used in a firstembodiment; and

FIG. 3 shows this first embodiment without the pumped option shown inFIG. 2, but with a dispersion compensating fibre unit equipped withmultiple distributed reflection elements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a silica optical fibre waveguide 1 typicallyexhibits dispersion which distorts transmitted signals havingsubstantial bandwidth. One previously proposed solution which is ratherinadequate is to operate at 1.311 m or at whatever wavelength aroundwhich dispersion is a minimum. Unfortunately minimum power loss occursat very different wavelengths from minimum dispersion. Generally,however, a length of dispersion compensating fibre 2 is required inseries with fibre 1. Thus, if the correct length of dispersioncompensating fibre 2 is inserted, an input pulse of waveform 3 may bebroadened to waveform 4 by the transmission fibre 1, and then thedistortion is compensated by the correct length of dispersioncompensating fibre 2 to recover the original pulse width and shape, asindicated by the approximately square pulse 3 ^(i) resemblingsubstantially the original waveform 3.

An option to compensate for the loss of power uses the fact that Ramanscattering increases with increasing germania concentration, so that aconventional dispersion compensating fibre, e.g. of highly dopedgermania silica, generates an amplified signal from input signals bymolecular scattering having a given frequency difference from a pumpfrequency. FIG. 2 shows this option in which waveguide 1 is compensatedfor dispersion by the use of dispersion compensating fibre 5. Becausethe dispersion compensating fibre 5 is lossy, it is pumped by means of adiode laser 6 at a power just enough to cause stimulated Raman gain at adownshifted frequency. Signals at this frequency are amplified, by thisRaman effect, and the energy of the pumping determines the amount ofRaman amplification. Thus losses in waveguide 1 can just be compensatedby Raman amplification in the dispersion compensating fibre 5, for thisspecific downshifted signal frequency corresponding to the Ramanfrequency shift.

In one typical sample of dispersion compensating fibre 5, we havecalculated that 100 mW of pump power is needed to compensate for the13.6 dB loss on an 80 kM section of the dispersion compensating fibre 5(the losses in the section of main waveguide 1 being separatelyaccounted for or compensated for). This calculation assumes a Raman gainof 10⁻¹² cm/w, and is somewhat conservative in assuming a low germaniaconcentration in the dispersion compensating fibre material. Assuming ahigher level of germania dopant concentration, to give a desired highdispersion (or need for only a smaller length), the Raman gaincoefficient would also increase, so that pump power could be decreasedfor the same amplification. The gain bandwidth for Raman amplificationis around 10 nm, and Raman amplifiers have the useful property that theygive quantum limited noise performance at any gain.

Such Raman amplifiers may be pumped at shorter wavelengths by arrangingfor intermediate Raman orders to oscillate in a resonator defined by theamplifier A. A second embodiment uses pairs of Bragg gratings in theside of the dispersion compensating fibre, each pair defining arespective cavity along the dispersion compensating fibre, such as tocause oscillations selectively at the respective other orders, andthereby to transfer a substantial amount of the power, at one or more ofthe unwanted orders, from the pump signal to the desired order to giveamplification to the signal. For instance, a diode-pumped Nd:YAG lasertransmits 1319 nm wavelength pumping power to a dispersion compensatingfibre such as that schematically shown at 5 in FIG. 2. If the requiredsignal amplification is to be at 1.55 μm (i.e. 1550 nm), at whichwavelength a standard dispersion compensating fibre is highlydispersive, then certain unwanted orders at 1380 nm and 1460 nm aregenerated by oscillations and are unrelated to the incident 1.5511 msignal. By the use of spaced grating pairs (not shown) reflective atrespective 1380 nm and 1460 nm, oscillatory cavities are set up whichtransfer the energy at these unwanted wavelengths to energy at a wantedwavelength. This option thus comprises a dispersion compensating fibreincluding oscillatory cavities defined by reflective gratings atunrequired oscillatory wavelengths unrelated to a required signalwavelength of operation, wherein all these wavelengths, required andunrequired, are or tend to be Raman signals generated by pumping thedispersion compensating fibre, e.g. by means of a standard diode pumpedNd:YAG laser, and the required signal is responsive also to an incidentsignal and is amplified enough to compensate for losses in theDdispersion compensating fibre and in line 1. The wanted signal can berecovered from downstream of the reflective grating pair or pairs (notshown).

An embodiment of the invention is described with reference to FIG. 3, inwhich the wanted amplified signal is derived by reflection from anoptical grating. In FIG. 3, the dispersion compensating fibre 8 iscaused to reflect the wanted signal from optical transmission line 1 toa circulator or other directional coupler 9 and thence to a detector orutilisation circuit 10; unwanted signals may be transmitted through athrough-path 11 of the dispersion compensating fibre 8 in FIG. 3.Alternatively all signals may be reflected at 11.

Amplification to compensate for losses if desired may be arranged as inFIG. 2 by pumping and by selected Raman molecular transitions. A problemwith dispersion compensating fibre is that of selecting the appropriatelength, desirably for economy's sake from a given length, whereby tointroduce the appropriate amount of compensation for the dispersioncaused by perhaps 50 kM or some unknown length of transmission waveguide1. One prior proposed way of selecting dispersion compensating fibrelengths is by cutting off sections, which tends to be inconvenient andwhich for WDM systems results in a compromise in throughput. Accordinglyby this embodiment of FIG. 3, reflection points are created at one ormore of the positions G₁, G₂, G₃ by means of Bragg gratings ofappropriate element spacing which determine reflection wavelength orfrequency, and of appropriate grating length which determines bandwidthof wavelength energy reflected, If G₁ is operative, the incident signalsat this frequency will be selectively reflected at this frequency, andwill traverse the first section, shown leftward of dispersioncompensating fibre 8, twice, introducing dispersion corresponding tothis double length, then leftwards to circulator 9 and branched toutilisation circuit 10. If other dispersions or different wavelengthsare to be selected, different reflecting gratings G₂ or G₃ are used atappropriate positions or appropriate grating element spacingsrespectively, either of these wavelengths being transmitted past gratingG₁ with negligible reflection. If grating G1 is likely not to berequired, it can be erased in non-destructive manner by heating orirradiation by UV. New radiation gratings G can be added, e.g. they canbe spliced in place as required. The position of a grating G determinesthe dispersion, and the spacing between grating elements in a grating Gdetermines the wavelength selected by reflection.

Raman amplification by pumping the dispersion compensating fibrematerial will usually be required, as described for FIG. 2, butoccasionally may be deemed unnecessary if the dispersion compensatingfibre 8 or optical waveguide line 1 does not introduce excessive powerlosses.

Variations of the arrangement of FIG. 3 may be used in furtherembodiments to separate energy at different wavelength in wavelengthdivision multiplexers (WDM). The two or more gratings G will be locatedby splicing in place or otherwise, at appropriate distances down thedispersion compensating fibre 8, to give the requisite equalisation forenergy at each of the different wavelengths.

Referring again to FIG. 3, when a Raman pump is used as in FIG. 2, afurther reflecting grating (not shown) can be located downstream of theother components of grating element spacing selected to reflect energyat the pump frequency. The reflected energy will reinforce the incidentpump energy to result in a higher overall gain.

Another alternative pumping scheme is to locate a pump source at bothends of the dispersion compensating fibre 5 of FIG. 2, when Ramanamplification is employed, thus again increasing gain when required.

What is claimed is:
 1. A dispersion compensation device including aplurality of wavelength or frequency dependent reflection devices eachof which is arranged to reflect signals at a predetermined wavelength,the reflection devices being located in series at predeterminedlocations along a Raman amplified dispersion compensating waveguideelement to reflect signals at the predetermined wavelengths for doublepasses through, at least a portion of, the waveguide, the location ofeach reflection device being so chosen as to cause the transmission of asignal at a specific wave-length through a predetermined length of thedispersion compensating waveguide element to compensate for dispersionin the signal and to equalise energy at each of the wavelengths, and acirculator or other directional coupler arranged to branch the reflectedsignal to a utilisation means.
 2. A dispersion compensation device asclaimed in claim 1 wherein each reflection device is a diffractiongrating.
 3. A dispersion compensation device as claimed in claim 2including fibre grating reflectors arranged to create optical cavitiesin the dispersion compensating waveguide element whereby energy atunwanted intermediate orders at undesired wavelengths can be transformedto energy at another wavelength, or can be dissipated.
 4. Use of thedispersion compensating device claimed in claim 1 wherein unwantedreflection devices are erased, and are replaced by at least onesubstitute reflection devices, in the dispersion compensating waveguidedevice, whereby different lengths of double-pass and hence differentdispersions can be selected.
 5. Use of the dispersion compensatingdevice claimed in claim 4 wherein unwanted reflection devices areerased, and at least one substitute reflection devices, which aregratings of selected element spacing, are spliced in the dispersioncompensating fibre device, so that different wanted wavelengths can beselectively reflected.
 6. An optical transmission system including atransmission path having first and second parts exhibiting dispersion,the dispersion of the second part compensating for the dispersion of thefirst part, wherein the second part includes a dispersion compensatingdevice as claimed in claim 1.